The present disclosure relates to power supply apparatuses that switch between a normal operation mode in which electric power is supplied to a plurality of loads including a first load, and a power saving mode in which electric power is supplied to a part of the loads including at least the first load, to supply electric power to the loads, and image forming apparatuses including the power supply apparatuses.
Image forming apparatuses, such as copy machines, printers, and facsimile apparatuses, operate in one of a normal operation mode and a power saving mode (also referred to as a sleep mode). The normal operation mode represents an operation mode in which the image forming apparatus is on standby so as to operate immediately when a print instruction is inputted. The power saving mode represents an operation mode in which the image forming apparatus is on standby so as to reduce power consumption of the apparatus by supplying power to a part of components. For example, in a case where a standby time period from power-on to input of a print instruction is shorter than or equal to a predetermined time period, the image forming apparatus operates in the normal operation mode. The image forming apparatus is controlled so as to shift from the normal operation mode to the power saving mode after elapse of the predetermined time period, in a case where the standby time period is longer than the predetermined time period. To date, image forming apparatuses have been known which halt, in the power saving mode, supply of power to controlled portions other than a main control portion, and also halt, when the standby state is further prolonged, supply of power to the main control portion, to enhance a power saving effect.
A power supply device for use in this type of image forming apparatus includes a power converter such as a DC/DC converter. The power converter converts commercial electric power supplied from the outside, to power suited for drive portions in the image forming apparatus, to supply the power to each drive portion. Specifically, AC100V commercial electric power is converted to DC power, and the voltage is increased or reduced to a voltage suited for each drive portion, to supply the power to each drive portion. However, in general, the conversion efficiency of the power converter is such that the higher the conversion efficiency is, the higher a ratio of a load current to an output current capacity of the power converter is, and the lower the conversion efficiency is, the less the ratio is. Therefore, in a case where power is supplied from a common power converter in each of operations of the normal operation mode and the power saving mode, since current consumption is low in the power saving mode, the ratio described above is extremely reduced. As a result, the conversion efficiency of the power converter becomes extremely low, and power saving effect may not be sufficient. Therefore, in conventional image forming apparatuses, a high-capacity power converter having a relatively high capacity, and a low-capacity power converter are separately provided, and power is supplied to each load from the high-capacity power converter in the normal operation mode, and power is supplied only to a part of the loads from the low-capacity power converter in the power saving mode.
In a case where the image forming apparatus receives a print instruction from the outside in the power saving mode, the image forming apparatus needs to shift to the normal operation mode. Therefore, power is supplied to an interface portion so as to receive the print instruction also in the power saving mode. Accordingly, an output current capacity of the low-capacity power converter used in the power saving mode is set based on a load current of the interface portion. In this case, the output current capacity of the low-capacity power converter is set so as to increase a rate of the load current of the interface portion, whereby conversion efficiency of the low-capacity power converter can be enhanced and power saving effect can be enhanced.
However, the load current varies among the interface portions. Therefore, when the output current capacity of the power converter is set in consideration of the variation, a ratio of the load current of the interface portion to the output current capacity may not be maximized, and high power-saving effect cannot be obtained. Further, an interface portion having a load current that is greatly higher than other interface portions may be incorporated in the image forming apparatus in some cases. When the output current capacity of the power converter is set in consideration of such a case, a ratio of the load current of the interface portion to the output current capacity is inevitably reduced. Consequently, in this case, the low-capacity power converter as described above cannot be used with an enhanced conversion efficiency, and power saving effect is insufficient. Further, various types of highly-functional electronic devices, for use in the image forming apparatuses, having high-degree of compatibility are distributed, and, for example, the interface portion of the image forming apparatus may be replaced with another compatible interface portion. In this case, in a case where a load current of the interface portion that has replaced the previous interface portion is higher than a load current of the previous interface portion, the load current of the interface portion may exceed the capacity of the power converter. Thus, the interface portion may malfunction or the power converter may be damaged. Such a problem may arise not only in the interface portion, but also in devices to which power is supplied when the image forming apparatus is in the power saving mode.